For three-dimensional objects, the illumination at an angle of incidence leads to the formation of shadows and thereby significantly increases contrast during viewing.
German Patent 28 52 203 discloses an illuminating arrangement which includes a planar holding ring having light conductor end faces with the holding ring being disposed in the tube of the objective symmetrically about the optical axis in an outer annular region. Light from the end faces of the light conductors exits therefrom parallel to the optical axis. This holding ring is displaceable in the tube so that the angle of incidence of the light can be changed by a movement of the holding ring. In this arrangement, it is a disadvantage that it is necessary to mechanically adjust the holding ring in order to obtain a change in the irradiation angle. The inner arrangement of the light conductor end faces makes it necessary to provide a very large ocular lens and furthermore leads to stray light in the tube. The use of many light conductors is also disturbing. These conductors must be brought in from the outside and their ends have to be precisely machined.
A ring-shaped illuminator for an ophthalmometer is disclosed in U.S. Pat. No. 4,666,269. In this ophthalmometer, a cylinder lens is provided which is made up of several concentric rings and has a plurality of circularly-shaped openings at its rearward end in the rearward focus. The ophthalmometer further includes a light source which can comprise a plurality of small light sources and the light of the light source is directed by the cylinder lens in parallel rays onto an eye with the center point rays meeting at a point on the optical axis. It is a disadvantage of this illumination that no selection of different illuminating angles or illuminating sectors is possible since the undirected light of the light source enters the lens through fixed diaphragms.
A multi-directional surface illuminator is disclosed in U.S. Pat. No. 4,567,551 wherein the light of each light source is deflected onto a zone plate via a mirror mounted at an angle. The mirror is arranged outside of the fitting holding the objective. The zone plate then focuses the light in a region surrounding the optical axis. With this surface illuminator, it is a disadvantage that the light enters from outside and that there can be no selection of the illuminating angle.
U.S. Pat. No. 4,706,168 discloses an illuminating arrangement wherein the light of a plurality of light sources is focused parallelly to the optical axis via two reflector rings into an object plane in a region around the optical axis. The light sources can be also limited to sectors and are disposed in a ring around the optical axis of an objective outside of the holder holding the objective. A change in the illuminating angle is possible by means of a mechanical adjustment of one of the two reflector rings. The significant disadvantage of this arrangement is the necessity of a mechanical adjustment in order to obtain a change of the illuminating angle. The reflecting surfaces are complex to process and this is a further disadvantageous characteristic of this illuminating arrangement.
Published German patent application DE-OS 37 34 691 discloses an illuminating arrangement for a microscope wherein a plurality of semiconductor light sources are arranged in two dimensions and therefore in a plane to form a planar light source. A control circuit is connected to the planar light source and, in association with the selection of the illuminating mode, permits the illumination of selected individual light sources or individual light source combinations. With reference to this planar light source, an optical illuminating system is arranged in alignment and collects the light emanating from the planar light source and directs this light from below onto a transparent specimen. The disadvantage of this arrangement is that it is only for transmitted light and therefore only suitable for transparent specimens. Furthermore, an optical illuminating system is required which is arranged in alignment with the planar light source and this leads to an illuminating arrangement which requires a very substantial amount of space.
An illuminating arrangement having a Lieberkuhn mirror is shown in the text entitled "ABC der Optik" by Karl Mutze, Verlag Werner Dausien (1961) on page 561. In this illuminating arrangement, the light radiated in around the object is focused by the mirror onto a region surrounding the optical axis. The light radiation is here parallel to the optical axis.